The Internet, which is conventionally used simply as a tool for data transmission, for example, for accessing websites or transmitting electronic mails, is recently applied also for voice communication services, which require a high communication quality for data transmission and a high real-time property. Among such voice communication services, VoIP (Voice over Internet Protocol) systems including IP (Internet Protocol) phone systems using Internet technologies have been put into practice along with the spread of the broadband.
VoIP is a transmission technology for transmitting and receiving voices via an IP network by use of the Internet Protocol (IP). A representative example of VoIP is an IP phone technology. An IP phone service provides voice communication using an IP network of a communication carrier. Communication carriers each have an IP network configured exclusively for the IP phone service in order to keep the quality of the IP phone service high. As described in, for example, Japanese Laid-Open Patent Publication No. 2009-272668, a user of an IP phone is assigned a phone number (e.g., number starting with “050”) for the IP phone by a communication carrier of the IP phone service. Such a user can perform voice transmission and receiving with another IP phone as well as with a landline phone using an existing public switched telephone network (PSTN).
However, with such an IP phone service, information on a transmission terminal which transmitted a conversation request signal may not be notified correctly to a receiving terminal, depending on the standards of the communication carriers or the specifications of the IP phone terminal which transmits or receives the signal. According to one alternative to the above-described system, each communication carrier acquires phone numbers in the PSTN and assigns one such phone number to each transmission terminal. However, with this system, there is a limit on the number of phone numbers which can be assigned because the cost for acquiring the phone numbers and maintaining the acquired phone numbers is high. That is, this system has a problem that the cost is increased as the number of users is increased.